Researchers provide evidence that the main source of the toxic metal is bullets from hunters that are left behind in animal carcasses, which the huge birds eat.

Field biologist Joe Burnett was standing at the base of a tree in Big Sur when the giant, limp body of a California condor landed at his feet.

Even before his team learned the official cause of death, they knew it was lead poisoning from the telltale signs they'd seen earlier in the bird, such as weight loss and erratic behavior.

"What happens to the condors as a result of lead poisoning — it's not pretty," said Burnett, who tracks California condors around the state for theVentana Wildlife Society.

Lead threatens all of the state's condors, according to a new study by Burnett and colleagues that was published online Monday in the Proceedings of the National Academy of Sciences. Of the 150free-flying condors in California during the period of the study, one in three had some amount of lead poisoning and one in five was severely poisoned each year, requiring intensive lead-removal treatments if the birds were to survive.

The study also provided evidence that the main source of the poison is lead bullets from hunters that are left behind in animal carcasses, which the birds eat.

For years, wildlife experts have called lead from ammunition a major threat to condors. But the new research shows how dangerous the toxic metal is to the future of the species, the authors said.

"The levels of lead are completely mind-boggling," said Donald Smith, an environmental toxicologist at UC Santa Cruz and senior author on the study. "Usually, in people, if we see an incidence of 1%we call it an epidemic — and this is 20%."

The California condor exists today only because of a series of remarkable interventions by conservationists.

The huge bird, which has a 9-foot wingspan and will fly 160 miles in a day to satisfy its hunger for rotting meat, once numbered more than 500 in the southwestern U.S. alone. By 1982, its population had plummeted to 22 due to lead poisoning, poaching and habitat destruction.

In 1987, all the remaining wild birds were captured for breeding and, five years later, the U.S. Fish and Wildlife Service began a program to release them back into the wild. Today there are more than 400 California condors, about half of which are flying free over California, Arizona and Mexico.

For many observers, this turnaround is a triumph of conservation. But that recovery is deceptive, the study authors said; it persists only because of intensive management of the birds.

Field biologists track the condors daily, and each is captured biannually for a thorough exam and blood tests. Often, Smith said, those tests turn up lead and the animals are then sent to the Los Angeles Zoo for a heavy-metal-removal treatment called chelation.

For years, researchers have been building a case that lead ammunition is poisoning the condors. The issue has pitted conservationists against hunters' rights groups such as the National Rifle Assn., which calls the connection "unfounded" on its website and has fought attempts to remove lead from ammunition. The NRA did not return a request for comment for this story.

In the new study, however, the authors said they connected the dots definitively.

The researchers collected the blood of every wild condor in California twice a year from 1997 to 2010. They analyzed the blood's lead content through a technique called stable isotopic analysis, which allowed them to determine whether the lead in the birds had the same mix of lead isotopes as the lead used in ammunition, compared with other potential sources such as paint.

The lead in the condors matched the bullets in all but nine of the 110 birds.

"This report really strengthens the case for bullets being the source," said J. Michael Scott, a zoologist at the University of Idaho inMoscow,who was not involved in the study.

A California state ban on lead ammunition in big-game hunting within the condor's habitat went into effect in 2008, largely due to efforts of condor conservationists.

But the ban left the use of lead ammunition in other types of hunting intact. And it had little effect on lead levels in the California condor population, the study found.

Environmental groups are now pushing for a complete ban on lead ammunition across the country. Earlier this month, seven conservation groups filed suit to force the Environmental Protection Agency to determine whether it has the authority to ban lead in ammunition.

Such a ban is feasible becausemost manufacturers now make copper bullets as well as lead ones, and those pose no risk to the birds, Smith said, adding: "There's just no reason to use lead-based ammunition anymore."

Myra Finkelstein, a toxicologist at UC Santa Cruz and the lead author on the study, said that without a broader ban the odds remain too high that condors will encounter lead-tainted food. If only one out of 100 carcasses has lead in it, each condor has a nearly one-in-two chance of being poisoned every year, according to the study's statistical analysis of feeding patterns.

"It only takes one exposure to poison a condor," Finkelstein said.

Smith said that despite the serious difficulties conservationists have had rehabilitating the California condor population, there are reasons to work hard to preserve the bird — not least the outsize role the bird plays in the public imagination.

But for Scott, the findings raise questions about the future of endangered-species management. There is a finite amount of money available to save endangered creatures, he said, and the condor is just one of more than 900 animals on the federal Endangered Species List that require active management to ensure their survival.

"If we were to walk away from the condor, it would quickly go extinct," he said. "So how long do we want to do this for — and what's the cost to other endangered species?"